Heterogeneity in Microseismicity and Stress Near Rupture-Limiting Section Boundaries Along the Late-Interseismic Alpine Fault

Paleoseismic evidence from the late-interseismic Alpine Fault suggests key section boundaries conditionally inhibit rupture. We utilize a year of data from a two-part seismometer network (Dense Westland Arrays Researching Fault Segmentation) to characterize ∼7,500 earthquakes (−0.7 ≤ MLv ≤ 4.2) and ∼800 focal mechanisms, producing high-resolution structural images of these boundaries to study effects of material and structural heterogeneities on mode-switching rupture behavior. Lithologically-controlled frictional behavior and crustal strength appear to influence lateral and vertical on-fault seismicity distributions. Ultramafic hanging-wall serpentinite and serpentinite-related fault core minerals along the South Westland (SW) boundary, result in a locally shallow seismogenic cuttoff (∼8 km) and abundant on-fault seismicity. Maximum horizontal compressive stress rotations (14° anti-clockwise and 20° clockwise near the SW and North Westland (NW) boundaries, respectively, relative to the Central Section), coupled with spatially variable fault frictional properties, are more important than geometry alone in controlling Sections' relative frictional stability. Whereas the SW and Central Sections are well-oriented for failure, the NW Section is severely misoriented compared with favorably oriented faults of the Marlborough Fault Zone, which possibly facilitate a preferred rupture route. Geometrically, a 40° dip change at the SW boundary may be accommodated either by a single through-going fault plane - a difficult geometry across which to obtain multi-segment earthquakes when considering rupture dynamics - or by a deeper vertical fault strand truncated by a shallower listric plane. Our new observations have implications for Alpine Fault rupture scenarios and highlight the need to consider a range of spatially heterogeneous, interdependent physical factors when evaluating controls on rupture segmentation.